Bioleaching of sulfide ores has been employed for many years in the extractive metallurgy industry. Successful bioleaching requires a thorough understanding of biological regimes and the role of each interrelating element therein. Enhancement of a bioleaching system requires not only a thorough knowledge of the interactive leaching elements, but also knowledge of tolerances for adjustments of those elements to effect the desired results. The goal of these processes is metal release or solubilization in an economically timely manner and concentration. Winning of the solubilizedmetal from a leach solution typically utilizes known extraction concentrationtechnology such as Ion Exchange (IX) or Solvent Extraction (SX). Leach solutions thus concentrated are made amenable to standard precipitation technology such as crystallizers and/or electrowiming (EW) production of metal cathodes.
Some common bioleaching reactants in a sulfide leach system are: acidophilic Thiobacillusferrooxidans and/or Thiobacillus thiooxidans; bacterial nutrients such as Med 64, Med 125, and PEGM, as disclosed in U.S. Pat. No. 5,413,624; oxygen (air) and carbon dioxide; oxidizers such as ferric sulfate, ferric chloride and ferric methane sulfonate; acids such as sulfuric acid and hydrochloric acid; and reaction catalysts such as silver and carbonaceous materials including, but not limited to, graphite, natural or synthetic, activated carbon or petroleum coke. Temperature is interactive to the bioleaching system, either ambient or elevated by devices.
The chemistry of the bioleaching system is complicated by the proper or improper management of the stated reactants. Two sets of reactions caused by an enhancement of a particular reactant are as follows: oxygen enriched leaching of chalcopyrite (CuFeS.sub.2) in the presence of a catalytic amount of silver as shown in U.S. Pat. No. 3,856,913, as illustrated by the reactions EQU 12CuFeS.sub.2 +51O.sub.2 +22H.sub.2 O.fwdarw.12CuSO.sub.4 +4H.sub.3 OFe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 +4H.sub.2 SO.sub.4 (1)
and if pyrite (FeS2) is present with the chalcopyrite: EQU 12FeS.sub.2 +45O.sub.2 +34H.sub.2 O.fwdarw.4H.sub.3 OFe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 +16H.sub.2 SO.sub.4 (2)
Both reactions generate sulfuric acid (H.sub.2 SO.sub.4) which can lower the pH of the reacted solutions to an intolerable level for acidophilic bacteria which might have been present in the reactants, thus suppressing the contributions which might have been derived from the bacteria.
Conventional chalcocite (Cu.sub.2 S) and covellite (CuS) bioleaching reactions, which are enhanced only by the presence of a catalyzing inoculum, are as follows: EQU (T.f.) BAC EQU Cu.sub.2 S+Fe.sub.2 (SO.sub.4).sub.3.fwdarw.CuS+CuSO.sub.4 +2FeSO.sub.4 (1) EQU (T.t.) BAC EQU CuS+Fe.sub.2 (SO.sub.4).sub.3.fwdarw.CuSO.sub.4 +2FeSO.sub.4 +S.sup.0 (2)
where Thiobacillus ferrooxidans serve to reoxidize the reduced ferrous sulfate (FeSO.sub.4) to ferric sulfate (Fe.sub.2 (SO.sub.4).sub.3), while the Thiobacillus thiooxidans catalyze the continued solubilization of the reaction (1) product covellite (CuS).
A negative reaction product of chalcocite/covellite bioleaching is elemental sulfur (Reaction 2). The negative effect of sulfur is that it forms an amorphous layer over the remaining (non-leached) covellite (Wan, R. V. et al. [1984] "Electrochemical Features of the Ferric Sulfate Leaching of CuFeS.sub.2 /C Aggregates," Office of Naval Research, Fed. Rpt. No. 36). This amorphous layer is impenetrableto the catalyzing T. thiooxidans, which limits or eliminates further solubilization reactions.
The above stated reactions illustrate only two of the numerous leach conditions which must be understood and properly managed to effect a predictable and desirable result. Further concerns of leach management are materials handling of the leach components, both solid and liquids, such as sulfide mineral concentrate solids and sulfuric acid and/or acidified ferric sulfate liquid reactants.
The bulk solids fed to a particular reaction process must meet the economic constraints of the selected system. Low grade (low metal content) sulfide ores are typically bioleached in dumps or lined heap piles, whereas low grade sulfide concentrates must be processed in low cost reaction vessels, such as passive vat leach tanks or, for higher grades, stir tanks or fluidized reaction vessels. Processing variables which further affect materials handling and processing costs are material feed size, whole ore or milled ore, metal content, solids retention time in reaction vessel (reaction rates), solid to liquid ratios, ie., slurry viscosities or pulp densities; and feed density, amenability of solids suspension (passive, stirring).
Recovery of metals from sulfide ores is often performed by first producing metal sulfide concentrates through flotation processes. Traditionally,the concentrates are smelted to drive off the sulfur and produce the metal. However, smelting is becoming environmentally unacceptable due to emissions of sulfur dioxide, and the quality of metals produced directly by smelting is inferior to that produced by leaching, solvent extraction, and electrowinning. Hence, there has been interest in processing concentrates by leaching, solvent extraction, and electrowinning.
Processes for refining of concentrates to the metal must meet economic constraints imposed by the marketplace. Stirred tank leaching of concentrates has several drawbacks, typically making it economically unviable. First, many concentrates are refractory to solubilization and require long residence times. Second, high power consumption is required for suspending and mixing of concentrates. In certain instances, stirred tank leaching of concentrates is performed under aggressive conditions using concentrated reagents and heating for higher value metals, but not for the base metals, such as copper and lead. Heap leaching of concentrates has also been investigated. However, processes for heap leaching of concentrates have encountered fluid flow problems such as channeling, ponding, and bypassing due to their fine particle size.
In areas of the world where there are few to no smelters, generally Western Europe, Africa, and parts of Asia, sulfuric acid is produced by burning sulfur. The plant needed to do this is capital-intensive. Operating costs are not much of a consideration due to the co-generation of electricity during the burning of sulfur which offsets the operating costs. A biological method to produce sulfuric acid solution from sulfur would be far less capital-intensive.